With a tendency towards the reduction in the size of patterns pertaining to miniaturization and integration of semiconductor devices, the photoresist layer and the patterns are getting thinner in order to prevent collapse of the photoresist pattern. The use of the thin photoresist pattern makes it difficult to etch a target layer, so an inorganic or organic layer with high etching resistance is provided between the photoresist pattern and the target layer to be etched. Such a layer is called “(resist) under-layer” or “hard mask”, and a process for patterning an under-layer using a photoresist pattern and then etching a target layer using the under-layer pattern is called “(resist) under-laying process”. The inorganic under-layer used in the under-laying process is composed of silicon nitride, silicon oxynitride, polysilicon, titanium nitride, amorphous carbon, and so forth, and typically prepared by the chemical vapor deposition (CVD) method. The under-layer produced by the chemical vapor deposition (CVD) method is excellent in etching selectivity and etching resistance but problematic in regard to particles or initial facility investment as required. To solve these problems, there has been studied on an organic under-layer that can be prepared by spin coating as a substitute for the under-layer prepared by deposition.
A multilayer resist comprising the organic under-layer typically has a dual-layer structure (using dual-layer resist technique) or a triple-layer structure (using triple-layer resist technique). For a resist having a dual-layer structure, the over-layer is a photoresist layer capable of pattern implementation, and the resist under-layer is a hydrocarbon layer available to the etching process using oxygen gas. The resist under-layer is supposed to play a role as a hard mask in etching an underlying substrate, so it is required to have high etching resistance and to consist of hydrocarbon alone containing substantially none of silicon atom in case of etching using oxygen gas. Further, the resist under-layer is needed to have a function as a layer for preventing scattered reflection of light source in order to control the standing wave of the overlying resist film and avoid collapse of the pattern when using a KrF or ArF light source. More specifically, it is necessary to control the reflection from the under-layer onto the resist over-layer.
For a resist having a triple-layer structure, an inorganic hard mask intermediate layer (i.e., a second under-layer composed of an inorganic substance) is further provided between the over-layer (i.e., a photoresist layer) and the resist under-layer (i.e., a first under-layer composed of a hydrocarbon compound). The second under-layer can be a silicon oxide layer, a silicon nitride layer, or a silicon oxynitride (SiON) layer as prepared by the chemical vapor deposition (CVD) method at high temperature. Preferably, the second under-layer can be a SiON layer which is highly effective as a bottom anti-reflective coating layer. The thickness of the second under-layer is 5 to 200 nm, preferably 10 to 100 nm. The resist under-layer (i.e., the first under-layer) is required to have thermal resistance at a temperature of 240 to 500° C., because the substrate is heated up to 240 to 500° C. in order to form the second under-layer (particularly, the SiON layer) on the resist under-layer. However, when the resist under-layer does is not resistant to such a high temperature, it is potentially susceptible to decomposition to contaminate the inside of the equipment during preparation of the inorganic hard mask intermediate layer (i.e., the second under-layer).